A technique used in communications for transmitting a number of separate signals simultaneously over a single channel or line. To maintain the integrity of each signal on the channel, multiplexing can separate the signals by time, space, or frequency. The device used to combine the signals is a multiplexer. Whenever there is a need to economize on transmission cost, there is a valid reason for use of multiplexing.

Imagine that you have several letters to take to the post office. You could get on to your Motor-bike or cycle and take one letter, mail it, come back home and get the second letter, take it to the post office and mail it, come again to get the third…… Silly way to do things!. Isn’t it? Why not take all the letters in the same trip, because they are all going to the post office. That is what multiplexing is: Using a resource (like your bike or cycle) to carry more than one message at a time.

 There are no inputs to a multiplexing. The multiplexer is connected by a single data link to a demultiplexer. The link is able to carry and separate channels of data. The multiplexer combines (multiplexes) data from the & input lines and transmits over a higher capacity data link. The demultiplexer accepts the multiplexed data stream, separates (demultiplexes) the data according to channel, and delivers them to the appropriate output lines.

The widespread use of multiplexing in data communications can be explained by the following.

1. The higher the data rate, the more cost effective the transmission facility. That is, for a given application and over a given distance, the cost per kbps declines with an increase in the data rate of the transmission facility. Similarly, the cost of transmission & receiving equipment, per kbps, declines with increasing data rate.

2. Most individual data communicating devices require relatively modest data rate support. For example, for most terminal & personal computer applications, a data rate of between 9600 bps and 64 kbps is generally adequate.


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